1. Field of the Invention
The invention relates to a method for manufacturing aluminum foils which are used as electrodes in electrolytic capacitors or condensers. The word "aluminum" in this specification defines and includes aluminum and its alloys.
2. Description of Prior Art
Aluminum foils are generally used as the electrodes in electrolytic capacitors, and before incorporated therein, subjected to chemical or electrochemical etching treatment in order to increase their effective surface and their capacitance per unit surface area. Simple etching treatment however cannot provide such foils with satisfactory capacitance.
High temperature heat treatment has widely been employed to be carried out at a temperature of about 500.degree. C. or higher during final annealing process after foil rolling process, so that "cluster texture" rich in "cubic azimuth" might be enhanced to the foils thereby improving their etching property. This method has also proved insufficient to meet the recent needs and demands for higher capacitance of electrolytic capacitors.
The present applicant proposed, as disclosed on the Japanese Patent Publication 58-34925, an aluminum material for electrolytic capacitors which material comprised an aluminum foil covered with a crystalline .gamma.-Al.sub.2 O.sub.3 coating of a given thickness and was improved in its etching property. The material were to be produced by forming at first a hydrated coating on the surface of aluminum foil. Subsequently, the aluminum foil covered with the hydrate coating was heated in non-oxidizing atmosphere at 500.degree. to 600.degree. C. for 2 to 10 hours. Said hydrate coating was thus converted to a crystalline .gamma.-Al.sub.2 O.sub.3 layer of 40 to 200 .ANG. in thickness.
This method has however not increased capacitance to a sufficient degree.